Question: Which of these numbers is prime? ${21,\ 63,\ 69,\ 71,\ 90}$
A prime number has exactly two factors: 1 and itself. The factors of 21 are 1, 3, 7, and 21. The factors of 63 are 1, 3, 7, 9, 21, and 63. The factors of 69 are 1, 3, 23, and 69. The factors of 71 are 1 and 71. The factors of 90 are 1, 2, 3, 5, 6, 9, 10, 15, 18, 30, 45, and 90. Thus, 71 is a prime number.